The objective of this project is to design, fabricate and evaluate a new, muscle-driven ambulatory assist system suitable for clinical testing in the home and community environments that maximizes the functional mobility of individuals with motor complete thoracic level spinal cord injury (SCI). Paralysis from SCI causes rapid degeneration of almost every major organ system. Commercially available externally powered robotic exoskeletons can begin to address such immobility in rehabilitation and supervised settings, but do nothing to counteract the disuse atrophy of the large lower extremity muscles and ensuing cardiovascular deconditioning. The maximal walking speeds and distances achieved with these devices fall far short of those necessary for safe and effective ambulation in the community. As a result, veterans with SCI are still unable to access many physical locations and life opportunities important for unrestricted reintegration into society. The ?hybrid? approach we propose is radically different from wearable walking robots. Our ?muscle first? strategy derives the primary motive power for walking and other maneuvers by eliciting relatively short bursts of high intensity contractions from the otherwise paralyzed muscles with electrical stimulation. Internalizing the primary power sources means the external components only have to lock/unlock the joints or shape the ballistic limb trajectories generated by the contracting muscles, thus eliminating the need for heavy motors at each joint and enabling users to reap the considerable physiological benefits of exercising their lower extremity muscles. The implanted neuromuscular component of our hybrid system is also continuously available for spontaneous exercise and short duration standing and stepping even without donning the external component. Stimulated contractions of the hip, knee and ankle muscles routinely generate sufficient power to maintain full weight bearing for several minutes, as well as to accomplish stepping motions for short distances without the need for powered exoskeletons. However, hip flexion can be inconsistent with stimulation alone, especially when attempting to climb steps or walk up ramps. We propose to augment stimulated contractions with a mechanical subsystem consisting of small, lightweight and efficient brace-mounted motors located at the hips. When powered by the contracting muscles, this novel configuration will stabilize the hips during stance, freely rotate during swing, and provide the low-level torques required to consistently achieve the desired limb movements in spite of variations in walking surfaces or stimulated responses. Since the motors only need to provide the incremental torques necessary to augment the stimulated hip muscles and shape the limb trajectories, the entire external structure can be significantly smaller, lighter, and quieter than commercially available powered exoskeletons based on a ?motor-first? strategy. Active knee extension will be generated by exciting the femoral nerve which routinely generates sufficient torque to stand and walk, while a similar mechanism to that proposed for the hip will lock during standing or mid-stance to rest the stimulated muscles, unlock during swing and stair ascent, and assist knee flexion immediately prior to swing. The mechanism will damp the impact of foot-floor contact, and gently lower the body during stair descent or transitioning from standing to sitting. A simple spring-assisted ankle brace will protect the foot and raise the toes during swing, while strong stimulated contractions of the calf muscles provide the propulsive power to drive walking at speeds far beyond those reported for existing exoskeletons. This project will define a practical clinical intervention to restore long-distance walking at near normal speeds suitable for daily activities and community use. After benchtop and laboratory testing, selected users will attempt to negotiate unrestricted community environments with the hybrid system. The proposed hybrid neuromechanical gait assist system should enable paralyzed veterans to return to healthy, productive and socially engaged lifestyles which will have significant impacts on quality of life and societal participation.